Theory of Phase Transitions in Molecular Crystals Applied to the Solid Hydrogen Halides

Abstract

Phase transitions in the hydrogen halide crystal which is composed of an array of linear molecules with nearest neighbor coupling are theoretically investigated by making use of the Frenkel model. The Hamiltonian is a sum of dipolar and quadrupolar interactions. The method of symmetry-breaking potential is applied with the use of an inverse temperature expansion. The results obtained are compared with those of the theory by Krieger and James, who investigated the orientational transitions with no attention to a particular substance by making use of the Pauling model. Depending upon the ratio of the coupling constants, we have a single second order transition, a single first order transition, two successive first order transitions, or a first order transition followed by a second order transition as the temperature lowers. A triple point is found, at which two different types of ordered phases and a disordered phase coexist.